In 1996 a workshop on Measurement of Oxidative Stress in Humans was held at NIEHS. There was a general consensus that it would be valuable to mount a study comparing various markers of oxidative stress measured from the same sample. One such marker is F2a-isoprostane which is quantitated by NICI-MS. Isoprostanes are prostaglandin-like compounds that are produced as a result of the chemical rather than enzymatic oxidation of arachidonic acid in membrane phospholipids. The isoprostanes have been shown to possess potent biological activity, and a number of studies have shown them to be accurate markers of lipid peroxidation in animal models of oxidative stress. The 8-iso-F2a-isoprostane has been the specific isomer quantified in most studies. The Mass Spectrometry Workgroup was asked to provide 8-iso-F2a-isoprostane analyses for NIEHS as part of the round-robin study of the various proposed biomarkers as well as to provide this capability to NIEHS scientists.